ABSTRACT The goal of this project is two-fold: 1) to determine how age compounds the effect of myocardial infarct (MI) on heart function and how this affects skeletal muscle function and exercise tolerance; and 2) to determine the ability of 2 deoxy-ATP (dATP) to affect heart and skeletal muscle performance, metabolism and exercise tolerance in age and MI induced heart failure. Most studies of performance decline with MI are done with young animal models, to look at infarct specific effects. However, MI occurs most often in the elderly, and it is not clear the extent to which this compounds pathologic effects at the system, organ and cell levels. Thus, we will study how age impacts the effects of MI on heart and skeletal muscle contractile and metabolic function. We will then use this model to study cardiac-specific vs. cardiac + skeletal muscle elevation of dATP. We have previously reported that dATP enhances contraction in demembranated cardiac and skeletal muscle by increasing myosin binding to actin (crossbridge formation) and crossbridge cycling. We have also reported that cellular levels of dATP can be elevated in the heart and skeletal muscle via transgenic or viral vector-mediated or over-expression of the enzyme Ribonucleotide Reductase (RNR). Both of these approaches enhance left ventricular (LV) heart function and increases the magnitude and speed of cardiomyocyte contraction & relaxation in normal hearts and rescues LV function of infarcted hearts of rodents and pigs. These previous MI studies were done in young adult animal and we did not determine how cardiac-specific elevation of dATP affected skeletal muscle or exercise tolerance. In preliminary data we demonstrate that 1) both demembranated cardiac and skeletal muscle from old mice have enhanced contraction when dATP (vs. ATP) is the substrate for contraction, 2) transgenic young mice with elevated heart and skeletal muscle [dATP] have greater exercise capacity, faster treadmill running and fatigue resistance, and 3) elevated cardiac RNR and dATP may protect against transition from an oxidative to glycolytic cardiac metabolic profile following MI (in young mice) and rescue this `more youthful' oxidative profile in old mice. In the proposed experiments we will determine if AAV-RNR vectors can improve cardiac and skeletal muscle performance, exercise capacity and metabolic performance of old mice with and without MI. We will compare vectors with cardiac-specific vs. striated muscle-specific expression of RNR. Thus our proposal offers a unique model to study how specifically targeting the heart to improve its performance can have secondary beneficial effects in skeletal muscle. Function will be measured at whole organ, cell and myofibril levels for both cardiac and skeletal muscle and coupled with measures of metabolic efficiency and activity, mitochondrial function, and metabolomics and proteomic analysis. This multi- scale analysis, using interdisciplinary approaches, will provide information for mechanistic interpretations. The results from these studies will determine feasibility of our approach (AAV-RNR mediated elevation of dATP in muscle) for treatment of heart failure and other age-related declines in cardiac function and exercise tolerance.